Lithium-air cells are nonaqueous cells that use oxygen in the air as a positive electrode active material. They have a theoretical stored energy per mass of a lithium metal as a negative electrode of as high as about 11140 Wh/kg, characteristically having a high energy density. Owing to such a high energy density, lithium-air cells are easily reduced in size and weight, and thereby they have attracted attention as high capacity cells that surpass the currently widely used lithium ion cells.
One example of such a lithium-air cell includes: an air electrode that contains an electrically conductive material such as carbon black, a catalyst such as manganese dioxide, and a binder such as polyvinylidene fluoride; an air electrode collector that collects current from the air electrode; a negative electrode that contains lithium as a negative electrode active material; a negative electrode collector that collects current therefrom; and a nonaqueous electrolyte solution.
Conventional nonaqueous electrolyte solutions for a lithium-air cell are prepared by dissolving LiPF6 or the like lithium salt in an organic solvent such as ethylene carbonate, propylene carbonate, dimethyl carbonate, diethyl carbonate, 1,2-dimethoxyethane, γ-butyrolactone, or tetrahydrofuran. However, such nonaqueous electrolyte solutions are disadvantageous in that the resulting lithium-air cell suffers deterioration of the organic solvent caused by oxygen radicals or the like produced during charge and discharge processes, and that the resulting lithium-air cell with open air holes in the casing of the cell, that is, with open holes to take oxygen in the positive electrode, shows a significant decrease in the discharge capacity due to effects of the ambient temperatures such as volatilization of the solvent of the electrolyte solution during use or storage under a high temperature environment. Nonaqueous electrolyte solutions for a lithium-air cell are also required to be in a stable liquid state without precipitation of salts or coagulation over a broad temperature range.
One known solution to these problems is to use a nonvolatile ionic liquid as the nonaqueous electrolyte solution. For example, Patent Literature 1 discloses use of an ionic liquid having a specific structure as a nonaqueous electrolyte solution of a nonaqueous electrolyte air cell.
However, though the use of an ionic liquid as a nonaqueous electrolyte solution of a lithium-air cell is preferable in terms of the non-volatility, the ionic liquid may be deteriorated due to oxygen radicals or the like produced in the charge and discharge processes of the cell. The ionic liquid also has such manufacturing problems as being expensive to produce as compared with conventionally used organic solvents and being difficult to purify.